As such an exchangeable head cutting tool, in, for example, Japanese Patent Application Laid-Open (kokai) JP-A-7-164234, there is proposed a cutting tool made such that a male thread portion is formed at a rear end portion of a cutting head integrally formed of cemented carbide and connection is made by screwing the male thread portion into a female thread portion formed in a holder. Further, Japanese Patent Application Laid-Open (kokai) JP-A-2003-251540 proposes another exchangeable head cutting tool wherein, a steel section is firmly fixed on a shank section made of cemented carbide by brazing; and in order to hold a cutting head to the steel section, the steel section provides a female thread portion on the back side from a position where the cutting head is mounted.
However, like the exchangeable head cutting tool described in Patent Citation 1, in a cutting tool in which a male thread portion is directly formed at a cutting head made of cemented carbide which has a high hardness but is also a hard brittle material, chipping easily occurs in threads, and thus it is not possible to reliably connect it to a holder. Further, in a holder in which a shank section made of cemented carbide and a steel section with a female thread portion formed therein are firmly fixed to each other by brazing, like Patent Citation 2, it is difficult to secure the strength of a joined portion between the shank section and the steel section and there is a concern that the steel section may be come off from the shank section due to cutting resistance.
Therefore, in Japanese Patent Application No. 2010-72765 and Japanese Patent Application No. 2010-218643, the inventors of the present invention previously proposed an exchangeable head cutting tool wherein:
its tool main body, such as a cutting head and/or a holder, made of a hard material for example cemented carbide, provides a mounting hole having concave portions in its inner peripheral surface, as if the peripheral surface were corrugated with predetermined surface roughness. In the above exchangeable head cutting tool, to join the tool main body and the connecting member together, a cylindrical mounting portion of a connecting member made of a metal material having a lower hardness than the above hard material is inserted into the mounting hole; the mounting portion is plastically deformed such that the diameter thereof is expanded, by press-fitting and extracting a press-fitting member (a punch) having an outer diameter larger than the inner diameter of the mounting portion into and from an inner peripheral portion of the mounting portion; and the outer peripheral surface of the mounting portion is brought into close contact with the inner peripheral surface of the mounting hole, thereby being engaged with the concave portions.
Therefore, in such an exchangeable head cutting tool, by forming a screw portion for connection of the cutting head or the holder in the connection member made of the metal material in which while a hardness is low, toughness is high and thus it is difficult to be damaged, and it is possible to reliably connect the cutting head and the holder. Further, since the mounting portion is plastically deformed by the press-fitting member such that the diameter thereof is expanded, and thus the outer peripheral surface thereof is brought into close contact with the inner peripheral surface of the mounting hole and also engaged with the concave portions, the joint strength between the tool main body and the connecting member is high and thus there is also no concern that the tool main body and the connecting member may be separated from each other due to cutting resistance and thus the cutting head may fall off.
However, in such an exchangeable head cutting tool, if in order to further enhance the joint strength between the tool main body and the connecting member, the connecting member is formed of a metal material having a lower hardness than the tool main body but having a relatively high hardness or the amount of plastic deformation of the mounting portion is increased by making a difference between the outer diameter of the press-fitting member and the inner diameter of the mounting portion large, excessive load acts on the press-fitting member and thus wear is promoted or the metal material forming the connection member is welded to the press-fitting member, whereby there is a concern that the life of the press-fitting member may be shortened. For this reason, it is difficult to mass-produce an exchangeable head cutting tool in which the joint strength between the tool main body and the connecting member is further increased in this way.
The present invention has been made under such a background and has an object to provide an exchangeable head cutting tool in which mass production is possible without causing shortening of the life of a press-fitting member even while further improving the joint strength between a tool main body and a connecting member.